1. Field of the Invention
This invention relates to a drilling method and to drilling apparatus. In particular, aspects of the invention relate to combined bore drilling and bore isolation methods and apparatus.
2. Description of the Related Art
In oil and gas exploration and production operations, subsurface hydrocarbon-bearing formations are accessed by drilling bores from the surface to intersect with the formations. Drilling is accomplished using a drill bit mounted on the end of a drill support member, commonly known as a drill string. The drill string may be rotated via a top drive or rotary table on a surface platform or rig, or a downhole motor may be mounted towards the lower end of the string. The drilled bores are lined with steel tubing, known as “casing”, which casing is cemented in the bore by filling the annulus between the casing and the surrounding bore wall with cement slurry. The casing inter alia supports the bore wall and prevents fluid flowing into or from the bore through the bore wall.
During a drilling operation it is normally the case that the drill string passes through an upper section of the bore, which is cased, and. a lower and more recently drilled bore section which is uncased. While drilling, it is not uncommon for the bore to intersect formations which create difficulties for the drilling operator, including: unstable formations which collapse into the bore; swelling formations which restrict the. bore and may trap the drill string in the bore; porous formations which result in loss of returning drilling fluid; and fluid-containing formations which result in uncontrolled flow of gas or liquid into the bore.
In some cases these difficulties may be overcome by, for example, pumping specialised fluids downhole to treat the problem formation. However, in other cases it may be necessary to retrieve the drill string and then run in casing or other bore liner to isolate the problem formation before drilling may recommence. Clearly, these operations will be time consuming and incur significant extra expense. Further, in the event of significant immediate problems, it may even become necessary to abandon the well.
In normal drilling operations1 the sequence of events in drilling and then casing a bore is similar, that is following drilling to a desired depth the drill string is retrieved and a casing string is then made up and run into the bore.
It is among the objectives of embodiments of the present invention to provide a method and apparatus which permit bore drilling and bore isolation operations to be executed in a single “trip”, that is a drill string need not be retrieved and a separate casing string run in prior to a bore lining or isolation operation being carried out.